The $p K_{a}$ values of the conjugate acids (in water) are: aniline 4.6, 4-nitroaniline 1.0, 3-nitroaniline 2.5, 2-nitroaniline $- 0.3$ . Which of these amines is the weakest base in aqueous solution?

Answer: A

A lower

p K_{a}

of the conjugate acid means a weaker base. The ortho isomer (

- 0.3

) is weakest: besides the

- I

and

- M

effects, the ortho nitro group hinders the solvation and stabilisation of the

- NH X_{3} X^{+}

group (ortho effect in anilines).